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Direct or from Storage.                        551

of two sheets of lead a and ^ rolled into a spiral, with insulating
strips between, and placed in a vessel containing diliite sulphuric
acid. Charging till the positive surfaces were coated with lead
dioxide and the negative with metallic lead, the plates were in
such a chemical condition as to constitute a return battery.
Faure shortened the time of charging by coating the plates with
red lead (the lower oxide), and covering this with parchment tied
with strips. The only difference in action was that spongy lead

was formed at the negative plate, thus giving a large surface.
Present storage or secondary batteries (otherwise accumulators)
are on Plante* or Faure's principle, and do not really store
electricity, but change electrical energy into that of chemical
separation. They are useful where the demand for power is
intermittent^ and are fairly effective, the leakage, during a few
days being but smaU. (Seepp. 958 and 1118.)

Efficiency.The work lost during transformation in a
dynamo may be as low as 8 %, though it more often reaches 15%
or 20 %. A*|redter loss usiialiy occurs, however, between generator